Preheat your oven to 350°F (175°C). Grease a loaf pan.
In a mixing bowl, whisk together flour, baking powder, and salt.
In another bowl, mix sugar, eggs, milk, melted butter, and vanilla extract until well combined.
Gradually add the wet ingredients to the dry ingredients, mixing until just combined.
Divide the batter into two bowls. Add cocoa powder to one bowl and mix well.
Spoon the batters alternately into the prepared loaf pan. Swirl with a knife for a marbled effect.
Bake for 50 minutes or until a toothpick inserted into the center comes out clean.
Let the bread cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.